1. Field of the invention
This invention relates to a process for producing a toner used in electrophotography, electrostatic recording, electrostatic printing or toner-jet recording.
b 2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, etc. are known as electrophotography, which is commonly a process in which, using a photoconductive material, copies are obtained by forming an electrostatic latent image on a photosensitive member by various means, subsequently developing the electrostatic latent image by the use of a toner to form a toner image, transferring the toner image to a recording medium such as paper as the occasion arises, and thereafter fixing the toner image by the action of heat, pressure or solvent vapor. As methods for developing the electrostatic image by the use of toners or methods for fixing the toner image, a variety of methods have been proposed, and methods suited for the respective image forming processes are employed.
In recent years, higher-speed copying, higher image quality and color image formation are required for the electrophotography.
Toners are commonly produced by melt-kneading colorants such as dyes and pigments into thermoplastic resins to effect uniform dispersion, followed by pulverization and classification using a fine grinding mill and a classifier, respectively, to produce toners having the desired particle diameters. This is a process known as a pulverization process.
Reasonably good toners can be produced by such a production process (pulverization process), but there is a limit to the range in which toner materials are selected. For example, colorant-dispersed resin materials must be brittle enough to be pulverizable with ease by means of an economically usable production apparatus. Since the colorant-dispersed resin materials must be made brittle to meet such a requirement, a group of particles having a broad particle size distribution tends to be formed when such a dispersion is actually pulverized at a high speed, especially causing a problem that fine particles having been pulverized excessively are included in this group of particles in a relatively large proportion. Moreover, such highly brittle materials tend to be further finely pulverized or powdered when used actually for the development in copying machines or the like.
Moreover, in toners produced by such pulverization, there are restrictions when release agents such as wax are added. More specifically, in order to disperse a release agent at a satisfactory level, (1) a certain degree of viscosity at temperatures where it is kneaded with resin must be kept and (2) the release agent must be in a content of about 5 parts by weight or less. Because of such restrictions, the toners produced by pulverization have a limit to their fixing performance.
In this melt-kneading and pulverization process, it is difficult to disperse solid fine particles of colorants or the like completely uniformly in the resin, and some toners may have a distribution in composition depending on the degree of dispersion to cause variations in developing performance of the toners. In addition, the resolution, solid-area uniformity and gradation reproducibility of images formed by toners commonly depends on the properties of toners, especially their particle diameter, in a large proportion, where the use of toners with a smaller particle diameter brings about images with higher quality. Accordingly, recently available printers and high-grade copying machines often make use of toners with a small particle diameter. However, in making toner particles have a smaller particle diameter by the pulverization process, about 5.0 .mu.m in volume-average particle diameter is the limit because of the ability of grinding mills.
To overcome this problem, a toner production process in which a polymerizable monomer composition having at least a polymerizable monomer is subjected to suspension polymerization (hereinafter "polymerization toner") is proposed (Japanese Patent Publication No. 36-10231). In this process for producing toners by suspension polymerization, a polymerizable monomer and a colorant, and also optionally a polymerization initiator, a cross-linking agent and other additives are uniformly dissolved or dispersed to form a polymerizable monomer composition. Thereafter, this polymerizable monomer composition is dispersed in a continuous phase (e.g., an aqueous phase) containing a dispersion stabilizer, by means of a suitable agitator, and is simultaneously subjected to polymerization reaction to obtain toner particles having the desired particle diameters. Since this process has no restrictions on the items stated in the pulverization process and has various advantages, it has lately attracted considerable attention.
More specifically, since this toner production process has no step of pulverization at all, this is a production process in which toner materials are not especially required to be brittle and also by which toners whose colorants may hardly stand bare to the surfaces of toner particles can be obtained. Moreover, in the polymerization toner, a release agent component can be encapsulated in toner particles, and hence the release agent can be contained in a-larger quantity than the toners obtained by pulverization. As to the dispersibility of a colorant, too, it does not especially come into question because the colorant can be dissolved or dispersed uniformly in the polymerizable monomer together with other additives. The process further has such an advantage that it is adaptable to making particle diameter smaller because any desired particle diameter and particle size distribution can be controlled by dispersion and granulation conditions.
However, even such a polymerization toner has problems to be settled, which are as discussed below.
In the polymerization toner, where various materials are dissolved or dispersed in a polymerizable monomer system to form a polymerizable monomer composition which is then suspended and dispersed in an aqueous medium, it is not necessarily easy in technical view to suspend and granulate polymerizable monomer composition particles stably in accordance with a combination of materials, conditions and so forth and also to complete polymerization reaction under stable conditions not causative of any coalescence of particles.
Especially in recent years, systems to which electrophotographic techniques are applied are advancing rapidly not only in conventional office-work copying machines but also in color copying and in the field of printers as output devices of computers. Under such circumstances, the process constitution of various systems has become great in variety, and physical properties of toner which are required concurrently therewith have become required precisely not only in respect of conventional items, i.e., particle size distribution, fluidity and triboelectric charging performance, but also in respect of the controlling of toner particle shape and toner particle surface properties.
Thus, suspension granulation and polymerization stability for the polymerization toner have a very great influence not only on productivity but also on physical properties of toners, and are important factors. Unstable suspension granulation and polymerization conditions may cause coalescence and agglomeration of particles to damage the particle size distribution and triboelectric charging performance greatly, so that it becomes impossible to control the toner particle shape and surface state (or profile).
In the past, for the purposes of, e.g., making suspension granulation stable, preventing particles from coalescing during polymerization and allowing resultant particles to have a sharp particle size distribution, many proposals have been made, as exemplified by a method in which the particle size distribution is controlled using a dispersant and an anionic surface-active agent in combination, disclosed in Japanese Patent Application Laid-open No. 57-42052, and a method in which particle size is controlled by adding an aqueous polymerization inhibitor, disclosed in Japanese Patent Application Laid-open No. 57-41649, Japanese Patent Publication No. 1-55643 and Japanese Patent Applications Laid-open No. 6-73101 and No. 7-165847. However, the former method has such a disadvantage that the surface-active agent may remain, which makes the triboelectric charging performance of toner particles unstable, resulting in a great lowering of developing performance of the toner particles. The latter method has such an advantage that by-product emulsion polymerization fine particles can be removed, but it has a problem beyond it, such that the method is not effective for the reduction of microsuspension particles having problems as fine particles. The presence of such microsuspension particles has the disadvantages of causing jamming of toner during development and non-uniform triboelectric charging.
Meanwhile, many proposals are also made so as to solve the problems the polymerization toner has. For example, proposed is a method in which, as disclosed typically in Japanese Patent Applications Laid-open No. 9-54457 and No. 7-49586, a dispersion stabilizer once produced is solubilized using an acid and thereafter again precipitated using an alkali to obtain a desired dispersion stabilizer under alkaline conditions so that polymerization toner particles having a sharp particle size distribution are obtained using such a dispersion stabilizer. This proposal, however, is insufficient under the present situation where the controlling of even the toner particle shape and surface properties is required, and can not satisfy all the required physical properties.
Japanese Patent Application Laid-open No. 7-301949 also discloses a method in which an aqueous sodium phosphate solution and an aqueous calcium chloride solution are mixed to form calcium phosphate directly in a dispersion medium. This method is a superior method, but the aqueous medium comes to have a pH of about 10 when the aqueous medium is produced by the method disclosed in this publication. When the polymerizable monomer composition having a polymerizable monomer, a colorant and a charge control agent is dispersed and granulated in the aqueous medium having such a pH, the colorant and the charge control agent tend to become decomposed, dissolved and changed in properties by the alkali. Hence, the additives such as colorants and charge control agent may become decomposed, dissolved and changed in properties depending on the time and temperature applied for the production of toner particles, making it difficult to produce toner particles having the desired charge control performance and coloring power. Also, once the additives such as colorants and charge control agents have dissolved partly, the uniformly dispersed state of the polymerizable monomer composition particles may be damaged to produce fine particles in a large quantity or cause agglomeration of particles, tending to make the particle size distribution of the resultant particles non-uniform. Especially in the case of the additives such as colorants and charge control agents, which are susceptible to alkalis, some may greatly become decomposed, dissolved and changed in properties by the alkalis to become unusable. Hence, in order that toner particles stable in physical properties and particle size distribution can be produced in an aqueous medium having an alkalinity, it has been necessary to control production conditions strictly and also there have been limitations on the additives such as colorants and charge control agents.
For the purposes of making particles free from any coalescence during polymerization and achieving uniform particle size distribution in a stable suspension system, Japanese Patent Publication No. 3-76749 discloses the proposal that a polymerizable monomer composition containing an anionic polymer is dispersed in an aqueous medium containing an inorganic acid and a dispersion stabilizer having an organic group with a nitrogen atom. This is considerably effective. However, since a strong alkali is used when the nitrogen-containing dispersion stabilizer is removed, the problem as discussed above have still remained unsettled.
Thus, in the production of polymerization toners, any effective production process has not been found which satisfies all the conditions that no coalescence of particles during granulation and during polymerization occurs, particles are present in a stable state throughout the reaction, the resultant toner particles have sharp particle size distribution and uniform triboelectric charging performance always stably and in a good reproducibility, the toner particle shape and surface state can be controlled, and no limitations on the materials used as toner materials may be imposed.